Abstract

The striated adductor of the Atlantic deep sea scallop (Placopecten magellanicus) a thick filament regulated muscle, contains little or no troponin We examined the effect on activation of two agents (MgATP and pH) that alter the contractile threshold of thin filament regulated muscle, presumably through effects on troponin, to see if they also alter that of thick filament regulated muscle. We find that decreasing MgATP from 2 to 0.1 mM shifts the force-pCa curve of chemically skinned scallop muscle to the left by about 0.8 log units (i.e. Ca2+ sensitivity increases some six-fold). Under similar conditions the force-pCa relation of frog skinned fibers shifts leftward by almost the same amount, 0.7 log units (Godt, 1974). The force-pCa curve of scallop was unaffected by a decrease in pH from 7 to 8.5. It is especially interesting because: A) the force-pCa relation of skinned fibers from frog (Robertson and Kerrick, 1979) and striated adductor of the Pacific scallop (Chlamys hastata hericia) (Donaldson, unpublished observations) is shifted to the right by about 0.5 log units over this pH range. Furthermore, B) decreasing pH is reported to decrease the calcium affinity of Placopecten myofibrils (Chantler et al., 1981). Thus the molecular details of thick filament regulation appear to be more complex and varied than hitherto supposed.